The subject matter described herein relates generally to stress sensing in ferromagnetic materials, and more particularly, to non-contact systems and methods for the sensing of stress in ferromagnetic materials.
Ferromagnetic materials have a magnetostrictive property that causes the materials to change shape in the presence of an applied magnetic field. The inverse is also true. When a force is applied to a ferromagnetic material, the magnetic properties, such as magnetic permeability, of the material change. A magnetostrictive sensor may sense the changes in magnetic permeability and, because the changes are proportional to the amount of stress applied to the ferromagnetic material, the resulting measurement may be used to calculate the amount of stress.
The changes in the magnetic permeability due to stress applied to the ferromagnetic material, however, may be small, making accurate measurement difficult. At least some known magnetostrictive sensors are used with a ferromagnetic material that has had a magnetic field permanently induced in at least a portion of the material to facilitate measuring the stress in the material. This step may require magnetically encoding the material to be sensed. Such an operation is typically expensive. In addition, such an operation makes it more difficult to retrofit existing systems with a magnetostrictive sensing system because the material to be sensed may have to be removed from the system to undergo permanent magnetic encoding, e.g., a shaft of a gas turbine engine. In at least some other magnetostrictive sensing systems, a temporary magnetic field is induced in a ferromagnetic material, and a magnetic field detector senses transient changes to the magnetic field. In such a system, steady state stress measurements may not be accurately measured or may be necessarily difficult to acquire.